1. Field of the Invention
The present invention generally relates to a disk drive, and more particularly to a method and system for detecting rotational vibration without using an external sensor, and more particularly by measuring the time shift of the magnetic transitions (e.g., time variations or jitters, or bit variations or jitters) due to vibration alone.
2. Description of the Related Art
As track density continues to grow, vibration induced track following error component becomes highly critical to the operation of a disk drive. The effective data rate throughput is degraded in the presence of rotational vibration. At high TPI (tracks per inch), the in-plane rotational vibration (theta coordinate) of a disk drive, directly impacts the TMR (track misregistration) component. A solution to this vibration challenge can be developed along several disciplines, ranging from novel mount systems to sophisticated sensors and servo algorithms.
A computer system may include one or more disk drives where each drive contributes to the total vibration environment. In addition, a computer system itself may be subjected to external seismic excitations. Head positioning accuracy in a disk drive is prone to both self-generated vibration and to vibration generated by a neighboring disk drive or other peripherals attached to the same mounting structure.
The present generation of 1.0″, 2.5″ and 3.5″ hard disk drives (HDDs) are designed to operate in portable and desk-top/server environments respectively. To reduce cost and weight of a computer system, manufacturers tend to fabricate the hard disk drive (HDD) mounting frame utilizing thin structural members. Therefore, a computer frame is a compliant object which makes it susceptible to vibration.
Such a mounting configuration makes a disk drive vulnerable to vibration excited by internal and external sources. An HDD with a rotary actuator system is highly sensitive to in-plane rotational vibration (RV) of its base plate.
As known, a head positioning servo system in an HDD performs three critical tasks. First, it moves the head to the vicinity of a target in a minimum time using a velocity servo under a seek mode. Next, it positions the head on the target track with a minimum settle-out time using a position controller without an integrating term in it. Finally, the servo system enters the track follow mode with a proportional-integral-derivative type (PID) position controller.
During the seek mode, maximum rotational acceleration torque followed by a deceleration torque is imparted by a voice coil motor (VCM)-based actuator. The corresponding reaction torque on the base-plate causes transient rotational vibration that can be detrimental to the positioning accuracy of the read/write heads.
However, the presence of random vibration impacts the track following precision, and to a lesser degree the settle-out performance. This problem has been recognized by the present inventors and the present inventors hereinbelow address the problem of random vibration as it critically affects the track following precision of an HDD actuator system.
Present 3.5″ disk drives have reached 40 kTPI, and in the future it is expected to grow beyond 50 kTPI. A major obstacle to raising the track density is inadequate head positioning accuracy in the presence of vibration disturbances. Due to exponential growth in TPI, positioning the read/write elements over a track has become a major challenge. Conventional servo control system requires continuous innovations to perform well under increasingly difficult operating conditions.
The mechanical components such as spindle motor assemblies are not perfectly mass-balanced, and during operation they produce harmonic vibration. Harmonic vibration produces both a linear and a rotational oscillatory motion of the entire HDD system. When not compensated, a track following error of 15% of the track pitch can be detrimental to a disk drive's “soft” and “hard” error rate performance. The positioning error due to this internally produced periodic vibration can be solved using a servo method proposed in U.S. Pat. No. 5,608,586, incorporated herein by reference.
By using special shock and vibration isolation mount design, the rotational oscillatory components due to internal spindle forcing can be minimized as taught by U.S. Pat. No. 5,400,196, incorporated herein by reference. However, a mount design optimized to decouple internal spindle vibration as covered by the U.S. Pat. No. 5,400,196, remains susceptible to external input vibration.
By deploying the isolation mounts along a polygon satisfying a particular set of criteria defined by Japan Patent No. 2,565,637, the external vibration inputs generating rotational vibration on an HDD can be minimized.
In U.S. Pat. No. 6,122,139, incorporated herein by reference, a method to neutralize the reaction by generating a counter torque using a secondary actuator is disclosed. An HDD with a novel sensing and control solution could provide an enhancement to the problem of random vibration.
By deploying dual PZT sensors 101, 102, as shown in FIG. 1, and a signal conditioning algorithm, a conventional system 100 (e.g., see A. Jinzenji et al. “Acceleration feedforward control against rotational disturbance in hard disk drives,” APMRC—Nov. 6-8, 2000, TA6-01-TA6-02) demonstrates a feedforward solution to random vibration. PZT sensors 101, 102 by themselves do not produce high quality output without additional innovation. FIG. 1 also illustrates a feedforward compensator 103, a conventional servo 104, a base plate 105, a head 106, a disk 107, and an actuator 108 for the head 106.
As shown in FIGS. 2A-2C, U.S. Pat. No. 5,721,457, incorporated herein by reference, shows a dual PZT configuration 201, 202 in a disk drive where the mass and inertia of the disk drive is exploited as the seismic body to measure angular and linear acceleration with substantial sensitivity.
That is, FIG. 2(a) illustrates a head disk assembly 200, FIG. 2(b) illustrates in greater detail the piezoelectric strain sensor 201, 202 for measuring acceleration, and FIG. 2(c) illustrates the head disk assembly 200 on a userframe 204 undergoing shock and vibration, with the dual PZTs 201, 202 providing an angular and linear acceleration inputs to a component 205, thereby resulting in a write inhibit signal being issued.
Another challenge in the use of PZTs is that they are sensitive to strain along multiple axes, and therefore they respond to vibration inputs in addition to the theta-dynamics.
To produce high fidelity signals in the range of 100-1000 Hz, the size of a PZT configuration must be large and such a design is not compatible with the electrical card height and manufacturing requirements in a disk drive.
On the other hand, reducing the PZT volume produces poor signal quality, and more particularly the signal drift in the low frequency range (˜100 Hz) is not easily stabilized. Signal stability and noise are key problems in employing a compact PZT configuration. Sudden drift in PZT signal can cause undesirable write-abort condition.
Use of dual PZTs further complicates the problem of matching the individual PZT gain and thermal sensitivity. With novel mechanical structures, the sensitivity of a PZT can be enhanced along the desired direction and minimized along the remaining directions. However, this requirement makes the sensor cost prohibitive for a disk drive application.
As an alternative approach using a capacitive sensing micromechanical device, C. Hernden, “Vibration cancellation using rotational accelerometer feed forward in HDDs,” Data Storage, November, 2000, pp. 22-28, attempts to produce a quality theta-acceleration sensor. Sensor size, bandwidth and cost are considered to be a limitation of a micro-electromechanical structures (MEMS) sensor.
Thus, prior to the present invention, there have been no optimized methods or structures for reducing the rotational vibration of the disk drive, in the plane of the rotary actuator which causes higher steady-state tracking error due to the finite bandwidth and gain of the servo position controller.
That is, the conventional techniques of using sensors such as dual piezoelectric (PZT) or electromotive force generator (EMF) to allow the servo position controller to compensate for this type of disturbance, have not been adequate or optimized. Indeed, such techniques have required an external sensor to detect the rotational vibration, thereby making the HDD costly and complex, and there has been no scalability with the expected future increase in tracks per inch (TPI) and bits per inch (BPI).